The present invention relates to a conductive roll used in an image forming apparatus of an electrophotographic system such as a copying machine or a printer and to a method of manufacturing the same. To be more specific, the present invention relates to a conductive roller used as a charging roll for charging a surface of an image carrier, as a developing roll for coating an image carrier with a toner, and as a transfer roll for transferring the toner from the image carrier onto a paper sheet, and to a method of manufacturing the same.
FIG. 2 shows how various rolls are used. As shown in the drawing, a transfer roll 1 and an image carrier roll 2 serve to collectively transfer the toner from the image carrier onto a paper sheet 3. A charging roll 4 for charging a surface of the image carrier and a developing roll 5 for coating the image carrier with the toner are arranged in the vicinity of the image carrier roll 2. Further, a pair of fixing rolls 6 are arranged downstream of the transfer roll 1.
Known is a conductive member formed of a high molecular weight elastomer or a high molecular weight cellular elastomer (sponge body) mixed with an electron conducting agent such as a metal powder, a metal oxide powder, whiskers or a conductive carbon black to allow the conductive member to exhibit a predetermined electric resistance. The conventional conductive member of this type is defective in that the conductive member is greatly dependent on voltage, that the electric resistance is rendered nonuniform depending on portions of the roll product, and that the electric resistance of the conductive member is gradually increased during a continuous power supply. However, the conventional electron conductive member is advantageous in that a difference in electric resistance as measured under a voltage of 1 kV is small between a low temperature-low humidity environment (temperature of 10.degree. C. and a relative humidity of 10%) and a high temperature-high humidity environment (temperature of 30.degree. C. and a relative humidity of 80%).
Also known is a conductive member formed of a high molecular weight elastomer or a high molecular weight cellular elastomer (sponge body) mixed with an ionic conducting agent such as inorganic ionic substances including lithium perchlorate, sodium perchlorate or calcium perchlorate, a cationic surfactant, an amphoteric ionic surfactant, or an organic ionic substance such as tetraethyl ammonium perchlorate (or butyl ammonium) to control the electric resistance of the conductive member at a predetermined value. The ionic conductive member of this type is defective in that there is a large difference in electric resistance as measured under a voltage of 1 kV between a low temperature-low humidity environment (temperature of 10.degree. C. and a relative humidity of 10%) and a high temperature-high humidity environment (temperature of 30.degree. C. and a relative humidity of 80%). However, the ionic conductive member of this type produces a merit, which is not produced by the electron conducting conductive member, that the voltage dependence, i.e., difference in electrical resistance produced when the voltage is changed, is low.
As described above, the conventional electron conducting conductive member containing an electron conducting agent such as a conductive carbon black or a metal oxide powder exhibits a high voltage dependence (i.e., the change in electric resistance caused by the change in voltage is large), resulting in failure to obtain a constant electric resistance. Therefore, when applied to, for example, a developing roll, the electron conductive member fails to obtain a predetermined amount of charge. As a result, the toner attached to the developing roll is rendered nonuniform in density, resulting in failure to obtain a high quality image.
Likewise, when the conventional electron conductive member is applied to a transfer roll, the nonuniformity in the resistance value of the electron conductive member causes the toner transferred onto the paper sheet to be nonuniform in density. It is impossible to obtain a high quality image in this case, too.
On the other hand, the ionic conductive member containing ionic conducting agent such as lithium perchlorate or a cationic ionic surfactant gives rise to a large difference in the electric resistance between a low temperature-low humidity environment and a high temperature-high humidity environment, making it difficult to obtain a constant electric resistance throughout the four seasons of a year. It follows that, when applied to, for example, a developing roll, a stable electric resistance cannot be obtained. To be more specific, the amount of charging is rendered highly nonuniform depending on the change in the environment. As a result, the developed toner is rendered unstable, leading to failure to obtain a high quality image.